Generally, while images are displayed on each pixel of an LCD panel using a thin film transistor for displaying with three primary colors such as R (red), G (green), and B (blue) colors, light leakage may occur among the pixels, influencing other pixels. As a result, sharpness may deteriorate. To solve the above problem, black matrices which cut off the light leakage while cutting off light to increase pixel sharpness are disposed among the pixels, thereby increasing sharpness of a screen. Besides, when light is incoming to a channel part of the thin film transistor, the channel part of the thin film transistor creates an optical current, causing a leakage current. The leakage current can be suppressed by locating black matrices in upper and lower parts of the thin film transistor.
However, if black matrix areas get wider, brightness may deteriorate even though screen sharpness increases. An opening ratio refers to a ratio occupied by a part for transmitting light on the entire screen of the liquid crystal display using the thin film transistor, and the opening ratio is an important factor when determining performance of the LCD panel. As a certain area gets increased, the opening ratio gets reduced.
FIG. 1 is a sectional view of a lower substrate of an LCD panel using a poly silicon thin film transistor by prior art. Referring to FIG. 1, the lower substrate of the LCD panel using the thin film transistor forms black matrices (2) by depositing and patterning an opaque film on a transparent substrate (1), deposits a first interlayer insulting film (3) on the transparent substrate (1) and the black matrices (2), and forms an active layer (4) consisting of a semiconductor film by depositing and patterning the semiconductor film on the insulating film (3). In addition, a gate insulating film (5) is deposited on the active layer (4). Contact holes contacted with the black matrices (2) are formed by opening the first interlayer insulating film (3) and the gate insulating film (5), and a first connection electrode (18) is formed by injecting a conductive material into the contact holes and patterning them. Also, a gate electrode (6) and a capacitor upper electrode (6′) are formed by depositing and patterning the conductive material on the gate insulating film (5). At this time, the first connection electrode (18), the gate electrode (6), and the capacitor upper electrode (6′) are made of the same material. Then, impurities are injected into the active layer (4) through an ion injection process after forming the gate electrode (6), and the active layer (4) is activated.
A second interlayer insulating film (7) is deposited on the first connection electrode (18), the gate electrode (6), the capacitor upper electrode (6′), and the gate insulating film (5). The capacitor upper electrode (6′), the gate insulating film, and the active layer configure a storage capacitor for maintaining data transmitted to a liquid crystal cell for a certain time.
Contact holes connected to the first connection electrode (18) and the capacitor upper electrode (6′), respectively, are comprised by opening a predetermined area of the second interlayer insulating film (7). Contact holes contacted with the active layer are formed by opening predetermined areas of the second interlayer insulating film and the gate insulating film at an interval of the gate electrode. By filling the formed contact holes with a metal material, a first common electrode (19) connected with the first connection electrode (18), a source electrode (8) contacted with a source area of the active layer, a drain electrode (8′) contacted with a drain area of the active layer, and a second common electrode (14) contacted with the capacitor upper electrode (6′) are formed, respectively.
A third interlayer insulating film (9) for covering the first common electrode (19), the source electrode (8), the drain electrode (8′), the second connection electrode (14), and the second interlayer insulating film (7) is deposited and planarized. Contact holes connected with the drain electrode (8′) and the second common electrode (14), respectively, are formed by opening a predetermined area of the planarized third interlayer insulating film (9), while forming a second connection electrode (10) connected with the drain electrode (8′) by filling the contact holes with a conductive metal and an upper black matrix (12) filled with a light cutoff material while being patterned.
Next, contact holes contacted with the second connection electrode (10) are formed after depositing and planarizing a fourth interlayer insulating film on the upper black matrix (12), the second connection electrode (10), and the third interlayer insulating film (9). Finally, a pixel electrode (14) is formed by depositing and patterning a transparent conductive film such as ITO to complete the manufacturing process.
However, even though the LCD panel using the poly silicon thin film transistor configured like above has many strong points, there still exist several problems to be solved. For instance, in a prior LCD element using a poly silicon thin film transistor, a big size of storage capacitor should be made to improve a capacity of the storage capacitor. In this case, the storage capacity has a problem of deteriorating an opening ratio of a screen by cutting off light of a backlight as well as black matrices, resulting in a deterioration of the opening ratio.
Particularly, since a recent LCD element using a poly silicon thin film transistor has a gradually reduced pixel size to realize miniaturization and high resolution, the size of the storage capacitor may exert a severe affect upon an opening ratio of the LCD element.